1. Field of the Invention
The present invention relates to well tools used in subterranean oil and gas wells under elevated conditions of temperature and pressure and, specifically, to the packing element systems employed on such well tools.
2. Description of the Prior Art
Downhole well tools of the type under consideration are used to maintain sealing integrity between inner and outer concentric conduits in subterranean wells. For instance, downhole well packers are commonly used to establish a seal in the annulus between the well casing and a smaller diameter production tubing string inserted into the casing. Permanent well packers can be set at a desired location within the well bore by means of mechanical tubing or wireline manipulation or by the use of hydraulic pressure. Once the permanent packer has been set at the prescribed location within the well, it can only be removed by milling or drilling the packer. Retrievable packers can be set by hydraulic or mechanical manipulation but can later be retrieved to the well surface.
Permanent packers are typically utilized at higher temperatures and pressures than comparable retrievable packers. Both permanent and retrievable packers are normally inserted within a well bore with adequate clearance between the packer and the well bore to avoid interference during installation. When the packer is set, radially expandable slips are actuated and move into engagement with the well casing. An annular seal or packing element, which is typically formed of a resilient, elastomeric material, is carried about a tubular mandrel and is expanded into engagement with the well casing in response to axial compression exerted on the packing element. The clearance between the packer mandrel and the expander surfaces and between the packer mandrel and the well casing provides an annular area into which the packing element, subjected to axial compression, can extrude.
In order to resist extrusion, permanent packers typically employ retaining or backup rings formed of a malleable metallic material. These rings are expandable into engagement with the casing upon the application of an axially compressive force sufficient to expand the packing element into the sealing engagement with the casing. The purpose of these rings is to seal off the annular clearance area to prevent extrusion of the packing material under extreme temperature and pressure conditions.
U.S. Pat. No. 3,109.493, issused Nov. 5, 1963, to Carter, shows a typical prior art retaining ring design. Although such extrusion preventing rings were effective in improving packer performance under extreme temperature and pressure conditions, several deficiencies continued to exist. Certain of the ring designs proved to be weak and would bend and fail under high loading. Other of the prior art ring designs allowed some flowing or extrusion of the packing material through the annular clearance between the ring and mandrel of the packer.
An object of the present invention is to provide a well packer having an improved extrusion preventing ring design which resists bending and deformation under extreme conditions of temperature and pressure.
Another object of the invention is to provide an improved ring design which more effectively fills the annular space between the packer mandrel and the well casing than did prior designs.
Additional objects, features and advantages will be apparent in the written description which follows.